In order to study geologic formations and materials located below the surface of the Earth, geophysical techniques are often utilized in order to indirectly determine or identify sub-surface formations. For example, seismic measurements taken on the surface of the Earth may be utilized to measure the physical properties of the subsurface Earth based on measurements of the propagation of low-frequency pressure waves that pass through and/or are reflected by different types of subsurface structures. Reflection seismology, seismic refraction, and seismic tomography are examples of such seismic identification techniques. The seismic measurements may be used to determine properties of the subsurface materials, along with the anomalies in these properties, in order to detect or infer the presence and position of ore minerals, hydrocarbons, geothermal reservoirs, groundwater reservoirs, and other geological structures.
Seismic reflection techniques are the most widely used geophysical technique for hydrocarbon exploration. For example, such techniques may be used to map the subsurface distribution of stratigraphy and its structure, which may then be used to determine the location of potential hydrocarbon accumulations. One reason seismic reflection techniques are so popular is because they provide a method for interpolating and extrapolating subsurface information over a large area.
Typically, to measure a seismic wave that has propagated through and/or was reflected by subsurface geologic structures, measurements are performed at the surface of the Earth to detect small, low-frequency ground displacement. The measurements that are performed at different times and/or locations may then be used to indirectly identify subsurface regions that have a higher probability of containing a desired material (e.g., ores, hydrocarbons, geothermal reservoirs, groundwater reservoirs, gas formations, etc.). For example, geophones are an example of a device that converts ground movement into a voltage in order to determine the amount of displacement.
Geophones may be passive analog devices that include a spring-mounted magnetic mass moving within a wire coil to generate an electrical signal. However, geophones often have difficulty operating at relatively low frequencies (e.g., 1-3 Hz and lower) due to their decreased sensitivity at these frequencies. This limitation in the range of the device may make distinguishing between signals of interest and ambient noise difficult. For example, since the frequency response of a geophone is typically that of a harmonic oscillator, and the corner frequency may be proportional to the inverse root of the moving mass within the geophone. Thus, geophones with low operating ranges on the order of 1-3 Hz are often impractical for deployment at the scale utilized for seismic imaging of large regions (e.g., they may be very large and cannot be practically used in large numbers over a large geographic region). Although the corner frequency may be lowered electronically, such techniques may introduce additional noise, may add additional processing complexity, may require higher power, and/or may increase the cost of the systems to impractical levels.
More recently, microelectromechanical systems (MEMS) have been utilized to operate at relative lower frequencies (e.g., perhaps as low as 3 Hz) for seismic exploration. MEMS devices may be accelerometers or geophones that utilize semi-conductive materials to perform acceleration or displacement measurements. However, due to the small size of MEMS devices and other factors, in order to detect the ground motion using MEMS devices an active feedback circuit may have to be utilized in order to measure the position of one or more small pieces of semi-conductive material within the device. As such, utilizing MEMS technology may require higher power operation while still lacking the capability to operate at desirable low-frequency ranges (e.g., 1-3 Hz and lower) in a commercially practical deployment.
Since these low frequency seismic waves may provide more information about subsurface formations that are found deeper within the Earth as compared to relatively higher frequency seismic waves, devices that are capable of detecting lower frequency information without the introduction of additional noise while still operating at relatively low power levels may be more desirable for many applications.